Periodontitis is a chronic infectious/inflammatory disease in which bacteria present in oral biofilm/dental plaque drive an inflammatory state leading to dysregulated remodeling and destruction of periodontal soft tissues and alveolar bone. This application will focus on mechanisms of inhibition of the generation of osteoclasts during inflammatory bone resorption. Cell activation and inflammatory mediator production in response to pathogenic oral bacteria is mediated by `pattern recognition receptors'that recognize conserved microbial molecules. Among the most important pattern recognition receptors are Toll like receptors 2 (TLR2) and TLR4 that have been implicated in driving inflammatory bone resorption in periodontitis. At the same time as they activate inflammation, TLRs induce potent homeostatic mechanisms to limit the intensity of inflammation and thus limit associated tissue damage. The extent of inflammation and the rate of associated bone loss in periodontitis are determined by the balance between inflammatory factors and the relative potency of feedback and homeostatic mechanisms that oppose inflammation and bone resorption, and promote healing. Thus, disease progression is evidence of relatively ineffective feedback inhibition that is not able to restrain inflammation and bone resorption, in contrast to effective homeostatic regulation that occurs during physiological resolution of inflammation or during quiescent phases of disease. Our overarching hypothesis is that augmenting physiological homeostatic mechanisms represents an effective approach to limiting bone resorption associated with inflammation. Therefore, we have initiated studies to understand molecular mechanisms that restrain osteoclast formation in an inflammatory setting such as periodontal disease. We have found that IL-10, a prototypic negative regulator that is induced by TLRs and widely expressed in periodontitis, suppresses osteoclastogenesis by inhibiting signaling by RANK and TREM-2, key receptors important for osteoclastogenesis. In addition, we found that TLRs directly suppress the formation of osteoclast precursors by inhibiting RANK and TREM-2 expression during development of osteoclast precursors. Interestingly, our results revealed qualitative and quantitative differences between the regulation of human and murine osteoclastogenesis, including differential regulation of TREM-2, that may help explain species differences in regulation of bone remodeling. In this application, we will investigate inhibitory/homeostatic pathways and molecular mechanisms by which IL-10 and TLRs inhibit human osteoclastogenesis. We will use human systems that are directly relevant for periodontitis pathogenesis and may reveal mechanisms that are not readily apparent in murine systems. We anticipate that our studies will yield insights into homeostatic regulation that can be exploited for therapeutic interventions to suppress alveolar bone resorption associated with periodontitis. Project Narrative: Periodontitis is a dental disease in which oral bacteria cause inflammation that destroys teeth. In this application we will investigate mechanisms that inhibit osteoclasts, the cells that destroy tooth bone in periodontitis. We anticipate that our studies will yield insights that can be exploited for therapeutic interventions to suppress the bone and tooth destruction associated with periodontitis.